1. Field of the Invention
The present invention relates to a developer for electrophotography, which includes a magnetic toner and a magnetic carrier.
2. Prior Art
Conventional developing methods using static latent images and toners are roughly divided into two main classes. One is a two component type developing method using a two-component developer essentially consisting of a non-magnetic toner and a carrier. The other is a single-component type developing method using a single component developer essentially consisting of a magnetic toner.
The developing method using a two-component developer including a non-magnetic toner and a carrier has the following drawbacks:
(1) there is a need for a toner density sensor to control the ratio of the toner and the carrier;
(2) the life of the developer is short; and
(3) a mixer for mixing the developer must be handled with care and a large developing machine must be present.
The single-component type developing method using a magnetic toner has the following disadvantages:
(1) an electrostatic charge element must be formed as a sleeve or a blade and has less electrostatic charge stability and capacity as compared with a carrier;
(2) there is a need for a precision developing machine to produce a uniform magnetic brush; and
(3) the magnetic toner has fewer transferring, fixing, and environmental properties and produces more damage to photo-conductor than a non-magnetic toner.
In order to overcome the disadvantages described above, two-component developers using magnetic toners and various magnetic carriers have been proposed, and some of these have been practical. Such two-component developers have advantages in that:
(1) various levels of toner density are acceptable and a density sensor is unnecessary;
(2) a triboelectric charging is good due to the use of a carrier;
(3) there is no need for precision in the operating of the developing machine as is necessary in the single component type developing method since a magnetic brush is easily formed;
(4) toner scattering is less than in the case of the two-component developer since magnetic material is included in the toner; and
(5) the developing unit can be as simplified as in the case of the single-component system due to the reduced need to stir the developer.
The two-component developing method using such a magnetic toner is proposed in U.S. Pat. No. 4,640,880 which discloses a developing method using a triboelectric magnetic toner and a ferrite carrier. In this method, the satisfied charge properties cannot be obtained in the case where toner ratio density is high in comparison with the usual non-magnetic toner employed in the two-component developer, i.e., the amount of the magnetic toner in the developer is not less than 20%. In addition, the image quality obtained by this method is not entirely adequate.
In the methods using magnetic toners, so-called incomplete copy images such as a b c, which may occur due to the decreased resistance of the toner and the failure of transferring, cannot be avoided. The magnetic toner is not acceptable with a minimum amount of this magnetic material. For this reason, it is difficult for the magnetic toner to maintain a toner resistance similar to the non-magnetic toner.
Hereinafter, some explanations will be given in connection with the so-called incomplete copy images. In the process of developing electrophotography, an image can be obtained by the steps of: developing a latent image on a photo-conductor with a toner on a sleeve to form a toner image; transferring the toner image on a paper sheet; and fixing the toner image to the paper sheet. Especially in the transferring step, the toner on the photo-conductor may not be uniformly transferred on the paper sheet and a part of the toner remains on the photo-conductor. As a result, the final transferred image may in part be missing. In particular, the center parts of the lines or dots of the image are frequently missing. An image having such a missing section is the so called "incomplete copy image".
In the developing method, the quality of the image is the most important consideration in practical use.